1. Field of the Invention
The present invention relates to a recording apparatus having sheet feed means for feeding recording media of different materials or a recording apparatus having a plurality of different types of sheet feed means.
2. Related Background Art
A recording apparatus which draws a light signal representing an input image information onto a photoconductor to form an electrostatic latent image, develops the latent image, transfers it onto a record sheet which serves as a recording medium, fixes it and ejects the record sheet out of the apparatus has been widely used.
In such a recording apparatus, some control to register the fed record sheet to the image developed on the photoconductor is required. For example, the feed status of the record sheet fed from the sheet feed means is detected to control an image write timing to write the image to the photoconductor so that the image is properly transferred to the record sheet.
On the other hand, the content outputted by the recording apparatus is versatile. It includes simple document information, image information, graphic information and the like as well as information recorded on record sheets of different recording media materials such as address printing or post card printing. Accordingly, a recording apparatus having sheet feed means such as a cassette as well as a manual sheet feed mechanism has been put into practical use.
In such a recording apparatus, in order to correct a skew of the record sheet, the record sheet is flexed in a loop shape at a timing roller (for example, a registration roller) and then it is fed in an image transfer direction (loop feed method). In this case, if the record sheet is a cut sheet having a thickness which is comparable to that of a conventional copy sheet, no problem is presented, but if it is a thick record sheet such as an envelope which has a small freedom of deformation, the envelope 75 fed by a feed roller 71 shown in FIG. 15 which serves as the record sheet may be bent in a snake shape when it reaches a registration roller 74 through transport guides 72 and 73, depending on the shape of the feed path.
In a recording apparatus which has a plurality of sheet feed mechanisms and transports a record sheet to the registration roller 74 through different transport paths, the record sheet may be snaked or not snaked depending on the transport path.
Where the transport path is curved, a time for the sheet to reach the timing roller changes depending on the type of sheet. When the sheet is thick, the reach time is significantly long. Accordingly, the occurrence of jam cannot be exactly detected by uniform timer monitoring.
In such a recording apparatus, an output timing of a vertical synchronous signal of the image is used as a reference of various control timings, the following problems (1)-(4) are presented.
(1) Where a size of a record sheet is not known, for example, an undefined size of sheet is used, or a sheet cut by a user and longitudinal and lateral dimensions of which are not known is used, an end timing of the image recording control cannot be specified and stop timings to drive units cannot be precisely controlled. In a recording apparatus which uses a known electrographic process, if the control is made by assuming a size which is equal to a maximum print sheet size that the recording apparatus can handle, the image recording control continues even after a rear edge of the sheet has passed, and the inside of the apparatus is contaminated by the scatter of toner. PA0 (2) Since it is necessary to continue the image recording control for the assumed length, a time to reach the end of image recording is unduely long. Similarly, automatic sheet ejection for a sheet remaining in the apparatus when the apparatus is powered on is also long. This is because the process time is unconditionally set to a time required from the start of the transport to the pass of the rear edge of the sheet through an exit of the transport path by assuming that a leading edge of the maximum sheet that can be handled by the apparatus is positioned at an entry of the sheet feed path. PA0 (3) Not only the sheet ejection time and the stop time of the drive unit are long in the above control, but also a temperature in the apparatus rises by the drive (because a duty factor of clutches increases), and a lifetime of the apparatus is shortened and a load for maintenance increases. As an example, an affect to a stay jam process of a sheet at a predetermined position is explained. PA0 (4) Even if the transport length of the sheet is detectable, the time from the start of the feed to the arrival at the exit of the transport path varies by a slip of rollers of the sheet transport system and the reduction of the transport speed due to the reduction of diameters by abrasion. Thus, when the rear edge of the sheet reaches the exit, errors have been accumulated and a margin is reduced. As a result, it is not easy to set the timing to detect the stay jam at the exit of the transport path.
The stay jam is detected when a sheet does not pass a predetermined point in the transport path at a time (within a certain margin) when the rear edge of the sheet is to pass that point. If the stay jam is detected based on the sheet transfer length of the maximum feedable sheet as described in (1) and (2) above, and if a sheet having a much shorter sheet transport length than the sheet transport length of the maximum feedable sheet is fed, the detection thereof is delayed and the sheet may be fed to a point where it is difficult to remove the sheet. Such sheet feed may damage the parts.